1. Field of the Invention
The present invention relates to a soldering technique employed for making electric and electronic parts or circuits using no lead. Particularly, it relates to a method for producing a tin-silver alloy plating film required to have solderability which is used as materials of electric and electronic parts, and to electric and electronic parts having the tin-silver alloy plating film formed by the above method.
2. Description of the Prior Art
Tin plating and tin alloy plating such as tin-lead alloy or tin-zinc alloy plating are widely used as industrial plating for parts in electronic industries, the representatives of which are weak current parts and lead frames, because of their good corrosion resistance and solderability. There have been developed various plating baths used for the above-mentioned tin plating and alloy plating such as tin-lead plating or tin-zinc plating.
Recently, environmental problems have been regarded as important, and, as to the parts used for IC packages, it has been attempted to use materials containing no injurious materials for environments. Of materials used for lead frames for electronic parts, one of the materials especially harmful for environments is lead used in solders. When allowed to stand, lead dissolves out of the solder and exerts a harmful influence upon the human body and living organisms. For this reason, development of solders or solder pastes using no lead has been tried in the electronic industries. However, materials having characteristics superior to the lead-containing solders have not yet been put to practical use. Various researches have been made on lead frames for electronic parts.
One example of conventional lead frames will be shown below. FIG. 3 is a plan view of a general lead frame, and FIG. 4 is a sectional view of a general lead frame. In FIG. 3 and FIG. 4, 1 indicates a portion on which a chip is mounted, 2 indicates an inner lead portion, 3 indicates an outer lead portion, 4 indicates a tie bar portion, 5 indicates a semiconductor chip, 6 indicates an adhesive, 7 indicates an electrode pad, 8 indicates a bonding wire, and 9 indicates a molding resin. Previously, the inner lead portion 2 is subjected to partial plating with Ag and the outer lead portion 3 is subjected to tin alloy plating. The semiconductor chip 5 is bonded to the portion 1 on which a chip is mounted, a terminal of the semiconductor chip 5 and the inner lead portion 2 are bonded with the bonding wire 8, and furthermore the whole other than the outer lead portion 3 is sealed with the molding resin 9.
The plan view of the lead frame shown in FIG. 3 shows only the lead frame before carrying out the bonding of semiconductor chip 5, the bonding with bonding wire 8, and the sealing with molding resin 9. Actually, after carrying out the bonding of semiconductor chip 5, the bonding with bonding wire 8, and the sealing with molding resin 9, the corresponding inner lead portion 2 and outer lead portion 3 are electrically connected, and the lead frame is cut so that the adjacent inner lead portions 2 per se and the adjacent outer lead portions 3 per se do not short-circuit each other. Thereafter, the outer lead portion 3 is bonded to other electronic part by soldering.
As substitutes for lead-containing solders, lead frames, the whole surface of which is plated with palladium, have been put to practical use. However, use of palladium as a single substance causes deterioration in wettability with the solder when heat is applied at the die attaching step or wire bonding step, and thus there is a problem of deterioration in reliability of soldering conducted for mounting. Therefore, there is proposed a lead frame for electronic parts which is thinly plated with gold as a protective film on the surface of palladium. However, countries supplying palladium are limited, and short supply thereof causes increase of price, resulting in problems in cost and stable supply. Furthermore, in the case of the lead frames, the whole surface of which is plated with palladium, a difference in potential is produced between palladium and a metal which is a substrate of the lead frame, and, therefore, nickel or a palladium-nickel alloy must be allowed to be present between the palladium and the substrate. In this case, if nickel or a nickel alloy or iron or an iron alloy is used as the substrate, there are problems in reliability owing to corrosion even if nickel or a palladium-nickel alloy is allowed to be present. Therefore, the palladium plating can only be applied to a substrate of copper or a copper alloy at present.
As for plating other than palladium plating, there is attempted to form a solder plating containing no lead using a metal such as indium, bismuth or zinc in place of lead used in the tin-lead based solders used at present. As solder alloys or solder pastes for reflow soldering, there are proposed ternary or quaternary alloys containing two or more metals in addition to tin. However, it is difficult to control the composition of the ternary or quaternary alloy in plating solution, and binary alloys comprising tin and another metal are mostly used, but alloys comprising tin and indium are difficult to put to practical use because indium is high in cost. In the case of alloys comprising tin and bismuth, the melting points can be reduced, but they are hard and brittle and, hence, inferior in workability and can hardly be used for lead frames which are subjected to flexural working. Moreover, they suffer from the problems that they are low in bonding strength, are inferior in thermal fatigue strength, and there is apt to occur the lift-off phenomenon in which the bonded part and the substrate is separated at the interface during surface mounting. Those which comprise tin and zinc have a melting point close to that of the conventional tin-lead based solders, and cost of zinc is low, but since zinc is readily oxidized in the air, when heat is applied in the step of fabrication of semiconductor devices such as IC, they are oxidized, resulting in deterioration of solder wettability.
Therefore, recently, alloys comprising tin and silver are proposed as the most hopeful alloys containing no lead, and intensive researches have been made to develop plating solutions. It has been attempted to improve characteristics such as appearance, flex cracking, and solder wettability and to inhibit discoloration due to thermal history, but these characteristics have not yet reached satisfactory levels at present.
For improving the solder wettability, it is effective to improve gloss of the plating film, but when the gloss is improved, the film increases in hardness and rather deteriorates in flex cracking characteristics. That is, it is extremely difficult to satisfy both the solderability and the flex cracking characteristics.
As mentioned above, the conventional technology can not necessarily be said to have sufficiently attained the practical task of solders using no lead. Particularly, in the case of lead frames of pre-plated frame (PPF) type in which the partial Ag plating of the inner lead portion on which a semiconductor chip is mounted and the tin alloy plating of the outer lead portion which is bonded to exterior electronic parts are previously conducted, solderability of the tin alloy plating is deteriorated due to the thermal history applied when the semiconductor chip is mounted, causing deterioration of solder wettability at the subsequent steps. This results in adverse effect on soldering performance of the outer lead portion externally exposed after sealing with molding resin.
Furthermore, when the outer lead portion externally exposed is subjected to flexural working at the time of surface mounting, if cracks are present in the portion which is bonded to the substrate, that portion becomes difficult to solder. As a result, formation of back fillet is insufficient, and there is an increased possibility of deterioration in bonding strength between the electronic part and the substrate.
Therefore, tin alloy plating materials used for the pre-plated lead frames must have higher performance and stable properties. Thus, improvement of tin alloy plating solutions has been made, but it is difficult to satisfy all of the required characteristics only by the improvement of plating solutions.
The tin-silver alloy plating films disclosed in JP-A-2000-307046 filed by the inventors have satisfactory physical properties as lead frames used for semiconductor apparatuses, namely, an initial zero-cross time of 0.35-0.8 second and a zero-cross time after heating (175xc2x0 C., 24 hours) of 0.45-0.9 second. The inventors have conducted further intensive research for providing tin-silver plating films excellent additionally in flex cracking characteristics. As a result, the present invention has been accomplished.
An object of the present invention is to provide a method for producing a tin-silver alloy plating film which can inhibit both the deterioration of solder wettability caused by thermal history and the deterioration of flex cracking characteristics. Another object is to provide a tin-silver alloy plating film which satisfies both the solder wettability and the flex cracking resistance. Further object is to provide an electronic part containing no lead which is one of environmentally harmful materials by using a lead frame for electronic part having the above plating film.
In order to attain the above objects of the present invention, according to the method for producing a tin-silver alloy plating film of the present invention, a tin-silver alloy plating film is formed by an electroplating method using a current having a pulse waveform of a current passing period of not less than 3 ms and not more than 500 ms and a current stopping period of not less than 1 ms and not more than 500 ms with a proviso that the stopping period is equal to or shorter than the passing period. It is preferred to use a current having a pulse waveform of a current passing period of not less than 3 ms and not more than 50 ms and a current stopping period of not less than 1 ms and not more than 50 ms with a proviso that the stopping period is equal to or shorter than the passing period. Preferably, the stopping period is shorter than the passing period. Direct current methods are mainly used for ordinary electroplating. According to the present invention, a current having the pulse waveform is used. The present invention uses a direct current obtained by rectification of an alternating current by a rectifier in a pulse waveform on the order of millisecond.
Furthermore, the tin-silver alloy plating film has a reflection density of not less than 1.0 and comprises an alloy containing silver and tin which has an orientation index of crystal at plane (220) of not less than 0.7 and not more than 5.4, an orientation index of crystal at plane (211) of not less than 0.8 and not more than 3.0, an orientation index of crystal at plane (200) of not more than 0.2 and an orientation index of crystal at plane (101) of not more than 1.4.
By employing the above construction, there can be provided a method for producing a tin-silver alloy plating film which is less in discoloration upon heating and satisfactory in solder wettability and flex cracking characteristics, the film and a lead frame for electronic parts having the film.